The present invention relates to a driving device and a driving method for an ink jet printing head which executes printing of letters, figures, images, etc. on objects such as paper, by changing the volumes of its pressure-generation chambers which are filled with ink by means of actuators (piezoelectric vibrators etc.) and thereby discharging minute ink drops from its nozzles corresponding to the pressure-generation chambers.
Various types of drop-on-demand ink jet printers have been proposed as disclosed in Japanese Publication of Examined Patent Applications No.SHO-53-12138, Japanese Patent Application Laid-Open No.HEI10-193587, etc. In a so-called drop-on-demand ink jet printer, the volume of a pressure-generation chamber which is filled with ink is changed (increased/decreased) by means of an actuator such as a piezoelectric vibrator, and thereby a minute ink drop is discharged from a nozzle corresponding to the pressure-generation chamber.
FIG. 1 is a schematic cross sectional view showing the composition of an ink jet printing head. Referring to FIG. 1, the ink jet printing head 1 includes a plurality of pressure-generation chambers 17, a nozzle plate 11 as a wall for the pressure-generation chambers 17, nozzles 15 which are provided to the nozzle plate 11 corresponding to each of the pressure-generation chambers 17 for discharging ink drops 19, a vibration plate 13, piezoelectric actuators 14 corresponding to each of the pressure-generation chambers 17 for vibrating the vibration plate 13 and thereby changing the volumes of the pressure-generation chambers 17 so as to cause the ink drop discharge from the nozzles 15, and the ink chambers 18 which are connected to the pressure-generation chambers 17 for supplying ink from an unshown ink tank to the pressure-generation chambers 17 via ink supply channels 16, a wall 12 disposed between the nozzle plate 11 and the vibration plate 13 to form the ink chambers 18.
In the ink jet printing head having the composition shown in FIG. 1, when the vibration plate 13 is vibrated by a piezoelectric actuator 14, the volume of a corresponding pressure-generation chamber 17 filled with ink is changed by the vibration, and thereby an ink drop 19 is discharged from a corresponding nozzle 15. The discharged ink drop 19 reaches the surface of an object such as paper and thereby forms an ink dot on the object. By repetition of the formation of the ink dots, letters, figures, images, etc. based on an image data are printed on the object.
The ink jet printing head 1 is required to discharge the ink drops 19 stably in order to realize precise printing on the object based on the image data. However, in actual printing by the ink jet printing head, desired printed output can not always be obtained since the ink drop discharge tends to become unstable due to various factors. One of the factors is evaporation of volatile ingredients of the ink.
The ink which is used for the ink jet printing generally includes water as the main solvent, and coloring agents (various organic dyes etc.) and surface-active agents are added to the main solvent. When such type of ink is employed, the water as the main solvent of the ink tends to evaporate from the surface of the ink at the opening of the nozzle 15 as an ink drop discharge pause period (that is, a period in which the ink drops are not discharged from the nozzle 15) becomes longer, thereby the ink becomes viscous or fixed partially, causing the nozzle 15 to stop up.
In order to resolve the ink fixation problem and maintain constant viscosity of the ink, xe2x80x9cink refresh operationxe2x80x9d is generally executed to the ink jet printing head, in which the ink jet printing head is withdrawn from the printing zone and the ink in the nozzles 15 is refreshed by means of forcible ink drop discharge, forcible ink suction by use of a pump, etc. However, in order to attain the stable ink drop discharge from the nozzles 15 and obtain high quality printed output, the above ink refresh operation has to be repeated frequently, thereby the consumption of the ink and printing cost are increased considerably and the need of handling and processing large amount of waste ink occurs.
Another method for avoiding the ink fixation problem without causing the increase of ink consumption has been disclosed in Japanese Patent Application Laid-Open No.SHO57-61576, in which a weak vibration, by which the ink drop discharge from the nozzle 15 is not caused, is applied by the piezoelectric actuator 14 to the vibration plate 13 and the pressure-generation chamber 17 also in the ink drop discharge pause periods.
FIGS. 2A through 2E are schematic cross sectional views of a nozzle of an ink jet printing head for explaining the above method disclosed in the Japanese Patent Application Laid-Open No.SHO57-61576.
Referring to FIG. 2A, ink 23 which is packed in the pressure-generation chamber 17 contacts air at the opening 21 of the nozzle 15.
Referring to FIG. 2B, at the interface (meniscus surface) 22 between the ink and air, evaporation of water in the ink 23 occurs and thereby a high viscosity ink layer 24 is formed near the meniscus surface 22.
When the weak vibration, which does not cause the ink drop discharge from the nozzle 15, is applied by the piezoelectric actuator 14 to the vibration plate 13 and the pressure-generation chamber 17, the meniscus exhibits small vibration as shown by the arrows in FIGS. 2C and 2D.
By the small meniscus vibration, the high viscosity ink layer 24 diffuses into the low viscosity ink 23 as shown in FIG. 2E, and thereby the viscosity of the ink in the pressure-generation chamber 17 becomes uniform. The reference character xe2x80x9c25xe2x80x9d shown in FIG. 2E indicates the ink having the uniform viscosity.
The above method is effective and applicable as long as the ink drop discharge pause period is short, however, the method can only slow the increase of the ink viscosity since the method does not execute replacement of the ink 23 in the nozzle 15. If the ink drop discharge pause period becomes long, fixation of the ink 23 in the nozzle 15 occurs eventually and thereby the following ink drop discharges become difficult or impossible.
In order to resolve the above problems, another ink jet printing method disclosed in Japanese Patent Application Laid-Open No.HEI9-29996 includes a xe2x80x9csmall vibration application processxe2x80x9d in which the small vibration (which does not cause the ink drop discharge from the nozzle 15) is applied to the pressure-generation chamber 17 in the ink drop discharge pause period and an xe2x80x9cink refresh processxe2x80x9d in which the ink jet printing head 1 is withdrawn from the printing zone periodically and the ink near the openings of the nozzles 15 and ink in the pressure-generation chambers 17 are forcibly discharged. In the ink refresh process, the amplitude of a refresh driving voltage waveform which is applied to the piezoelectric actuator 14 for the forcible ink drop discharge for the ink refresh operation is set larger than that of an discharge driving voltage waveform which is employed for ordinary ink drop discharge for printing, thereby discharge of large amount of ink and replacement of ink in the pressure-generation chamber 17 are conducted so as to avoid the fixation and bodying up of the ink for the long term.
However, in the above ink jet printing method of Japanese Patent Application Laid-Open No.HEI9-29996 employing the combination of the small meniscus vibration and the ink refresh operation (forcible ink drop discharge), the ink refresh operation (ink refresh process) is executed according to a fixed refresh operation cycle. The fixed refresh operation cycle has to be set short so as to be able to cover the severest possible condition within the operation-guaranteed ranges of the ink jet printing head 1. Therefore, also in the ink jet printing method of Japanese Patent Application Laid-Open No.HEI9-29996, ink consumption is necessitated to be large and printing speed is necessitated to be lowered due to the ink replacement (ink refresh operation).
Further, the size of the ink drop is becoming smaller and smaller these days due to the recent market requirements for high quality printing. As the ink drop size becomes smaller, even a little variation of the ink viscosity affects the print quality. For example, due to an ink drop discharge pause period of about 2 seconds under normal temperature and humidity, a fall of discharged ink drop speed of approximately 2 m/s occurs when the ink drop diameter is 25 xcexcm, while the fall is only 0.5 m/s when the ink drop diameter is 40 xcexcm. Therefore, the frequency of the ink refresh operation (forcible ink drop discharge) has to be increased as the size of the ink drop is made smaller, thereby the consumption of the ink also increases.
It is therefore the primary object of the present invention to provide a driving device and a driving method for an ink jet printing head, by which stable discharge of ink drops (especially, minute ink drops) and high quality ink jet printing can be maintained even if the ink drop discharge pause period continued long, without causing the large ink consumption and the long printing time due to the ink replacement (ink refresh operation).
In accordance with a first aspect of the present invention, there is provided a driving device for an ink jet printing head which has a plurality of pressure-generation chambers filled with ink, a plurality of nozzles corresponding to the pressure-generation chambers from which the ink in the pressure-generation chambers are discharged, and a plurality of driving means corresponding to the pressure-generation chambers for causing change of pressure in the pressure-generation chambers and thereby letting ink drops be discharged from the nozzles onto an object. The driving device for an ink jet printing head comprises a vibration application means and an ink refresh operation means. The vibration application means applies vibration, which does not cause the ink drop discharge from the nozzle, to the pressure-generation chambers corresponding to nozzles which are not executing ink drop discharge. The, ink refresh operation means periodically executes ink refresh operation for removing the ink in the nozzles and replacing the ink with fresh ink, according to a refresh operation cycle which is appropriately set based on the temperature and/or humidity measured around the ink jet printing head.
In accordance with a second aspect of the present invention, in the first aspect, the ink refresh operation is executed by means of forcible ink drop discharge from the nozzles.
In accordance with a third aspect of the present invention, in the first aspect, the ink refresh operation is executed by means of ink suction by a pump.
In accordance with a fourth aspect of the present invention, in the first aspect, the ink refresh operation means includes a refresh information storage means, a temperature/humidity measurement means, a refresh operation cycle setting means and an ink refresh operation execution means. In the refresh information storage means, data concerning appropriate refresh operation cycles corresponding to various temperature and/or humidity around the ink jet printing head are stored. The temperature/humidity measurement means measures the temperature and/or humidity around the ink jet printing head. The refresh operation cycle setting means reads out appropriate refresh operation cycle data from the refresh information storage means based on the temperature and/or humidity measured by the temperature/humidity measurement means, and sets the refresh operation cycle according to the refresh operation cycle data read out from the refresh information storage means. The ink refresh operation execution means periodically executes the ink refresh operation according to the refresh operation cycle which has been set by the refresh operation cycle setting means.
In accordance with a fifth aspect of the present invention, in the second aspect, the forcible ink drop discharge from the nozzles is executed by use of a refresh driving voltage waveform which is generated by a special-purpose refresh waveform generation circuit.
In accordance with a sixth aspect of the present invention, in the second aspect, the forcible ink drop discharge from the nozzles is executed by use of a discharge driving voltage waveform which is generally used for the discharge of ink drops of the largest size for ordinary printing.
In accordance with a seventh aspect of the present invention, in the second aspect, the forcible ink drop discharge from the nozzles is executed after withdrawing the ink jet printing head from the object.
In accordance with an eighth aspect of the present invention, in the first aspect, the refresh operation cycle is set shorter as the temperature around the ink jet printing head gets lower.
In accordance with a ninth aspect of the present invention, in the first aspect, the refresh operation cycle is set shorter as the humidity around the ink jet printing head gets lower.
In accordance with a tenth aspect of the present invention, in the first aspect, the refresh operation cycle is set shorter as the size of the ink drops discharged from the nozzles becomes smaller.
In accordance with an eleventh aspect of the present invention, there is provided a driving method for an ink jet printing head which has a plurality of pressure-generation chambers filled with ink, a plurality of nozzles corresponding to the pressure-generation chambers from which the ink in the pressure-generation chambers are discharged, and a plurality of driving means corresponding to the pressure-generation chambers for causing change of pressure in the pressure-generation chambers and thereby letting ink drops be discharged from the nozzles onto an object. The driving method for an ink jet printing head comprises a vibration application step and an ink refresh operation step. In the vibration application step, vibration which does not cause the ink drop discharge from the nozzle is applied to the pressure-generation chambers corresponding to nozzles which are not executing ink drop discharge. In the ink refresh operation step, ink refresh operation, for removing the ink in the nozzles and replacing the ink with fresh ink, is periodically executed according to a refresh operation cycle which is appropriately set based on the temperature and/or humidity measured around the ink jet printing head.
In accordance with a twelfth aspect of the present invention, in the eleventh aspect, the ink refresh operation is executed by means of forcible ink drop discharge from the nozzles.
In accordance with a thirteenth aspect of the present invention, in the eleventh aspect, the ink refresh operation is executed by means of ink suction by a pump.
In accordance with a fourteenth aspect of the present invention, in the eleventh aspect, the ink refresh operation step is implemented by a refresh operation cycle data storage step, a temperature/humidity measurement step, a refresh operation cycle setting step and an ink refresh operation execution step. In the refresh operation cycle data storage step, data concerning appropriate refresh operation cycles corresponding to various temperature and/or humidity around the ink jet printing head are stored in a refresh information storage means. In the temperature/humidity measurement step, the temperature and/or humidity around the ink jet printing head is measured. In the refresh operation cycle setting step, appropriate refresh operation cycle data is read out from the refresh information storage means based on the temperature and/or humidity measured in the temperature/humidity measurement step, and the refresh operation cycle is set according to the refresh operation cycle data read out from the refresh information storage means. In the ink refresh operation execution step, the ink refresh operation is periodically executed according to the refresh operation cycle which has been set in the refresh operation cycle setting step.
In accordance with a fifteenth aspect of the present invention, in the twelfth aspect, the forcible ink drop discharge from the nozzles is executed by use of a refresh driving voltage waveform which is generated by a special-purpose refresh waveform generation circuit.
In accordance with a sixteenth aspect of the present invention, in the twelfth aspect, the forcible ink drop discharge from the nozzles is executed by use of a discharge driving voltage waveform which is generally used for the discharge of ink drops of the largest size for ordinary printing.
In accordance with a seventeenth aspect of the present invention, in the twelfth aspect, the forcible ink drop discharge from the nozzles is executed after withdrawing the ink jet printing head from the object.
In accordance with an eighteenth aspect of the present invention, in the eleventh aspect, the refresh operation cycle is set shorter as the temperature around the ink jet printing head gets lower.
In accordance with a nineteenth aspect of the present invention, in the eleventh aspect, the refresh operation cycle is set shorter as the humidity around the ink jet printing head gets lower.
In accordance with a twentieth aspect of the present invention, in the eleventh aspect, the refresh operation cycle is set shorter as the size of the ink drops discharged from the nozzles becomes smaller.